A virus is sub-microscopic infectious particle composed of proteins coated with nucleic acid. a virus is cellular and requires a living host to survive. it causes many diseases to host, which causes an immune response. virus have no cell wall as well as no ribosomes are present in it.

the protein coat is present instead of the cell wall. It is a small infectious agent that harms only inside of the living cell of living organism. the nucleus is also absent in it, the structure of the virus as DNA or RNA enclosed in protein coated. they are smaller in size, the viruses are very harmful to living beings they even destroy our brain system. But they are very useful for Genetic Engineering.